I have both X3 and DX50. IMO they sound nothing similar to each other. To me, DX50 takes the upper hand.
X3 is darn good a player for its price. The sound is geared towards the warmer side of things with slight emphasis on mid bass which can be distracting at times. Vocals seem to take a front standing too. Very nice vocals here. Details and separation is there, but not in your face. I would say it's slightly laid back or take a back seat compared to the vocals. Treble has a very smooth presentation which is portrayed as laid back and relaxing too. In general, X3 is non fatiguing. Black or dark background is superb on X3. Probably the player may gain another step with the next release of its firmware in March? Will have to wait and see..
DX50 has got better transparency and clarity. Mids, details and separation as opposed to X3 is more prominent. Vocals isn't as upfront, but very engaging too. Seems to me that the soundstage is wider as well. Lows focus more on sub bass and is much tighter and more controlled. Treble as compared to X3 is definitely more present but very smooth as well. Coupled with the latest firmware, it's a winner in my book.
Overall, both X3 and DX50 has a different sound signature and shares more differences than similarities.